1. Field of the Invention
The present invention relates to a process for forming a deposited film adapted for use as an amorphous or crystalline functional film for example as a semiconductor, insulator, conductor or metal, in particular for an active or passive semiconductor device, an optical semiconductor device, a solar cell or an electrophotographic photosensitive device.
2. Description of the Prior Art
A deposited film can be formed for example by the vacuum evaporation, plasma CVD, thermal CVD, photo CVD, reactive sputtering, ion plating methods etc., among which the plasma CVD method has been employed both widely and commercially.
However the deposited film obtained by these methods still has room for improvement. In particular, for achieving application to electronic devices of higher performance, in electrical and optical properties, fatigue and other environmental characteristics in repeated use, and in productivity and mass productivity including uniformity and reproducibility, deposited films need improvement.
The reaction process involved in the formation of deposited film by conventional plasma CVD methods is considerably more complex than that for the so-called thermal CVD method, and is still unclear in various aspects. Also the formation of deposited film relies on a number of parameters such the substrate temperature, flow rate and ratio of introduced gases, pressure during film formation, high-frequency electric power, electrode structure, reactor structure, gas exhaust rate, plasma generating method etc. The plasma may become temporarily unstable to endow significant undesirable properties to the formed film. Besides it has been difficult to generalize the manufacturing conditions since certain parameters have to be determined specifically for each apparatus employed.
Among the aforementioned methods, the plasma CVD method is regarded as best as it can provide an amorphous silicon film with electrical and optical properties meeting the requirements of various applications.
However, in certain applications of the deposited film, the formation thereof by the plasma CVD method not only requires a large investment for the manufacturing apparatus but also necessitates complex process controls with narrow tolerances and delicate adjustments, in order to achieve reproducible mass production with a large area, a uniform film thickness and uniform quality.
On the other hand, the conventional ordinary CVD technology not only requires a high temperature but also is unable to produce, on a commercial basis, a deposited film with satisfactory characteristics.
It has therefore been desired to develop a process of forming a functional film on a large scale with an inexpensive apparatus while maintaining practical properties and uniformity.